Crust of the Earth: A Symposium
The elastic-strain rebound characteristic of the aftershock sequence of the Kern County, California, earthquake of July 21, 1952, was of the dual type first observed in the Long Beach earthquake of 1933. Availability of precise epicenters for this sequence brought to light the fact that the two components of the characteristic are derived separately from the two sides of the fault. Aftershocks which produced the compressional creep strain release occurred on the southeast side of the fault while those which generated the shearing creep strain release occurred on the northwest side with a delay in onset of 37 hours.
Tectonic activity near ϕ = 34° south, λ 58° east, exhibited by the Indian Ocean sequence of shallow earthquakes all having epicenters within an area of a few square degrees, has continued in its original form. Following the earthquake of October 24, 1933, the sequence was interrupted for a number of years. By extrapolating the original curve it was possible to predict the magnitude of the accumulated strain available at any subsequent date. Thus in 1948 there was available sufficient strain to produce a 7.5 magnitude earthquake. On October 7, 1949, a small shock of 6.5 magnitude occurred, and this was followed on December 8, 1951, by a shock of magnitude 7.6, which brought the observed curve to coincidence with the extrapolated curve within the limits of observational error. The rate of increase of strain in this region is now so low that it will require another 60 years to produce a 7.5 magnitude earthquake.
A further study of the regional earthquake sequences occurring along the Pacific arc suggests that the principal orogenic structure responsible for each of the great linear and curvilinear mountain ranges and oceanic trenches is one of two types of complex reverse faults, designated oceanic and marginal. The oceanic faults situated within the oceanic domain extend from the surface to depths of approximately 700 km with an average dip of 61°. The marginal faults along the continental margins occur in dual and triple forms. The dual faults have a shallow member extending from the surface to a depth of 60 km and an intermediate member extending to a depth of approximately 300 km with an average dip of 33°. The triple form has a third component extending down to a depth of approximately 650 km with a dip of 60°. In the continental domain the 300-km level appears to represent a tectonic discontinuity which may be the lower boundary of the continents.
The elastic-strain rebound characteristic of all world shallow earthquakes magnitude 8 and larger indicates that the great Assam earthquake of 1950 marked the close of a cycle of activity, which began some time before 1904 when precise observations first became available. On the basis of the small segment of the new cycle now perceptible, it appears that the rate of world-strain generation has increased.